The molecular mechanism of lead inhibition of human porphobilinogen synthase.

Abstract

Human porphobilinogen synthase (PBGS) is a main target in lead poisoning. Human PBGS purifies with eight Zn(II) per homo-octamer; four ZnA have predominantly nonsulfur ligands, and four ZnB have predominantly sulfur ligands. Only four Zn(II) are required for activity. To better elucidate the roles of Zn(II) and Pb(II), we produced human PBGS mutants that are designed to lack either the ZnA or ZnB sites. These proteins, MinusZnA (H131A, C223A) and MinusZnB (C122A, C124A, C132A), each become purified with four Zn(II) per octamer, thus confirming an asymmetry in the human PBGS structure. MinusZnA is fully active, whereas MinusZnB is far less active, verifying an important catalytic role for ZnB and the removed cysteine residues. Kinetic properties of the mutants and wild type proteins are described. Comparison of Pb(II) inhibition of the mutants shows that ligands to both ZnA and ZnB interact with Pb(II). The ZnB ligands preferentially interact with Pb(II). At least one ZnA ligand is responsible for the slow tight binding behavior of Pb(II). The data support a novel model where a high affinity lead site is a hybrid of the ZnA and ZnB sites. We propose that the lone electron pair of Pb(II) precludes Pb(II) to function in PBGS catalysis.

• Authors: E K Jaffe, J Martins, J Li, J Kervinen, R L Dunbrack Jr
• Journal: The Journal of biological chemistry (J Biol Chem) Vol. 276 Issue 2 Pg. 1531-7 (Jan 12 2001) ISSN: 0021-9258 [Print] United States
• PMID: 11032836 (Publication Type: Comparative Study, Journal Article, Research Support, U.S. Gov't, P.H.S.)

Chemical References

• Recombinant Proteins
• Lead
• Porphobilinogen Synthase
• Zinc

Topics

• Amino Acid Substitution
• Binding Sites
• Humans
• Hydrogen-Ion Concentration
• Kinetics
• Lead (pharmacology)
• Models, Molecular
• Mutagenesis, Site-Directed
• Porphobilinogen Synthase (antagonists & inhibitors, chemistry, genetics)
• Protein Conformation
• Recombinant Proteins (antagonists & inhibitors, chemistry)
• Sequence Deletion
• Zinc (metabolism)